The basic incandescent light bulb dimmer has existed as a commercial product for over 40 years (hereinafter lamp and light bulb are used interchangeably). The lamp dimmer became practical with the development of a class of semiconductor solid state switch devices known as thyristors that were introduced to the electronics markets in the early 1960's. Prior to that time, such dimmers consisted of rheostats, a form of variable power resistors.
Rheostats dimmed an incandescent lamp by transferring a selectable percentage of power from the lamp to the rheostat. Rheostat dimmers generated substantial heat, limiting their use. The use of thyristor dimmers generated dramatically less heat. These solid state devices provided efficient dimmers by rapidly switching power to a lamp on and off in a prescribed manner to efficiently dim the lamp. The solid state thyristor switch was fully on or off, generating little heat thereby improving efficiency. This sharp reduction in heat made possible the commercialization of popular wall-mounted lamp dimmers that are commonly found in homes.
Over the last 40 years, hundreds of patents have been issued on incandescent bulb dimmer circuits and their physical design characteristics. During that time, the basic and common circuit approach to lamp dimmers has continued to be a technique known as phase control. This technique is commonly found in controllers for lamps, heaters and motor speed controls (drills, saws, electric cars) and is well understood by those skilled in the art.
U.S. Pat. No. 3,896,334 ('334) illustrates the use of thyristors. This patent is incorporated herein by reference.
In known applications, a resistor and capacitor are connected in series to form a charging circuit for the capacitor. During each half cycle of the AC line voltage (the power source), the capacitor charges towards the line voltage. However, at a predetermined voltage, the capacitor discharges, triggering a triac into conduction, thereby turning on the lamp by applying the full AC line voltage to the lamp. The triac turns off, thereby turning off the lamp, on each half cycle when the AC voltage returns to about zero volts. The triac noted here is a known type of thyristor which exhibits bi-directional or bi-lateral switching characteristics. Such a triac is described in “The General Electric, SCR Manual” fifth edition. This manual is referred to as Reference 1.
This charge and discharge of the capacitor occurs each half cycle, with the capacitor being essentially reset at the end of each half cycle. The ratio of on to off of the triac and lamp, each half cycle, determines the average power delivered to the lamp and therefore sets the brightness (and the dimming) level. Persistence of vision associated with the human eye makes the switching on and off of the lamp each half cycle imperceptible. Such use of a phase control circuit to control lamp illumination is very well known and the principles of such circuits are described in many patents and in Reference 1.
In an adjustable lamp dimming device, the resistor portion of the charging circuit is typically a potentiometer, thereby allowing the user to vary the RC time constant involved and thereby the time to reach the capacitor discharge point. In a fixed illumination application, the triac or other thyristor type device turns on at a predetermined point, and the potentiometer can be replaced by a fixed resistor. U.S. Pat. Nos. 3,836,814 and 4,547,704 describe use of such a fixed resistor value, and these patents are incorporated herein by reference.
In other applications, a resistor, capacitor, triac and a diac (another semiconductor switching device also described in Reference 1) form a dimming circuit. These devices can all be replaced by a single thyristor device called a sidac which, on each AC power half cycle, senses the amplitude of the AC line voltage and exhibits a controlled avalanche into full conduction (fully on). Avalanche is a well known term in the art. U.S. Pat. No. 4,980,607 describes such a design and is incorporated herein by reference. Teccor Div. of Littlefuse Corp., Thyristor Product Catalog and Application Notes, published in 2002, herein after Reference 2, describes the theory of operation of a sidac.
In known dimming circuits, particular resistors or potentiometers, capacitors, thyristos, diacs and their operating specifications are well known to those skilled in the art. In ordinary dimmers there is usually a mechanical on/off switch whereby the lamp is turned off regardless of the setting of the dimmer. Such a mechanical switch is not further discussed herein.
While conventional wall mounted lamp dimmers have become a commonplace, economical commodity, derivatives of this technology are now also being built into the lamp or the lamp fixture itself. Typically these built-in electronic devices perform as life-extending devices rather than as lamp dimmers since these electronic devices are not easily accessible.
Consequently in a given lamp application, there may be two circuits in series: one to extend lamp life (while minimally dimming the lamp), and a second circuit that provides a range of brightness control (dimming). In such a situation undesirable interaction, instability and other anomalous operations can occur due to these two circuits. Typically, flicker, erratic, and non-linear dimming occurs. For example, if a light bulb has a built in (via its socket or cord) switching circuit meant to extend the bulb life and an external dimmer circuit (like a wall mounted dimmer), the dimmer circuit, rather than seeing a low resistance charging path of the bulb filament only, will see an off switch, e.g. an off thyristor. An off thyristor might exhibit an equivalent resistance of over several megohms. In such a condition, the timing capacitor in the wall-mounted dimmer may take more than a few AC line half cycles to charge, thereby upsetting the normal discharge/reset mechanism in the dimmer. Dimmer mechanisms are intended to switch on allowing portions of each AC line half cycle to reach the bulb filament. Such disruption of the normal discharge/charge mechanism can result erratic light behavior, like visible flickering and diminished range of dimming control.
In the commonly used dimmer circuits described above, a capacitor charges through a resistor and the low resistance lamp filament. However, in the above mentioned case where a wall-mounted dimmer and a life extending semi-conductor device are combined in series with the bulb filament, the capacitor will not charge in the normal fashion. The off life extending semiconductor will cause erratic operation of the combination.
It is an objective of this invention to minimize such undesirable effects as described above, thereby providing compatibility between life extending devices and ordinary light dimmers whenever they are used together.